1. Field of the Invention
This invention relates to the determination of non-uniformity compensation (NUC) terms for optical imagers, and more particularly to a low-cost method of calibrating precision NUC terms based on a moving target.
2. Description of the Related Art
Optoelectronic imagers such as Focal Plane Arrays (FPAs) in the IR, near visible, visible or other bands detect incident radiation and convert it to electrical signals to record an image of a scene. The response of the imager on a pixel-by-pixel basis can change dramatically and non-uniformly over time and based on environmental and operating conditions. These non-uniformities appear as fixed-pattern noise in the recorded images. The purpose of non-uniformity correction; known as ‘calibration’ if done off-line typically during manufacture or as ‘compensation’ if done on-line just prior to use of the imager, is to reduce the fixed-pattern noise.
Although arbitrary order non-uniformity compensation (NUC) terms can be computed to correct for non-uniformities, the terms of most interest are typically the offset (0th order term) and the gain (1st order term). The offset terms are relatively unstable, hence are typically compensated in the field just prior to or as the imager is being used. Gain terms are relatively stable and thus are typically calibrated offline, usually at the time of manufacture. In some systems, the gain terms may be ‘tweaked’ just prior to use. Known techniques for compensating both the offset and gain terms are based on the premise that on-average all pixels should see the same value.
The predominant approach for compensating the offset terms uses a blurred version of the scene created optically, through motion of the imager, or through temporal averaging in the field. Based on this premise, any high spatial frequency components that are detected in the blurred image for each pixel are deemed to be the result of non-uniform pixel response. The blurred image is corrected to remove the high frequency components. The same correction is then applied to the subsequent non-blurred image. This approach is serviceable for relatively “flat” imagery but struggles with scenes which contain significant content at high spatial frequencies. These may be perceived as non-uniformities and “compensated” producing scene and body-motion dependent artifacts.
The predominant approach for calibrating the gain terms is to expose the imager to uniform flood sources at different temperatures. The response of each pixel is taken as the difference value between these two sources to first cancel pixel offsets. A gain term is then calculated for each pixel to flatten the apparent response over the entire imager. The offset terms are discarded but the gain terms are saved to calibrate the imager. Different sets of gain terms may be calibrated for different operating conditions of the imager. A problem with this approach is that the flood measurements are conducted in a separate vacuum test chamber from the moving target tests performed with the calibrated imager. Providing multiple vacuum test chambers and moving the imager between test chambers significantly increases test time and cost.